Many prior efforts have been made toward deterring or preventing thefts in the nature of shoplifting, or other undesired removal of "contraband" articles or goods, for example, unchecked library books or the like, and such prior efforts have given rise to a variety of different systems and approaches, based upon different technological phenomena including, for example, detection of permanent magnet pieces, a variety of electromagnetic field applications, microwave systems, infrared or ultraviolet, etc. These rather extensive prior efforts have, quite understandably, advanced the general state of the art in these different fields, and have in general enhanced the degree of success available; however, the desired end is exceedingly difficult from a technological point of view, since the areas to be monitored (in general, doorways or like points of egress) are large in a physical sense, whereas the articles under surveillance are usually relatively small, requiring a proportionally tiny detection element or "marker". Generally speaking, this requires exceedingly high system sensitivity, but it is not only important to detect the illicit passage of contraband material; it is almost equally as important to avoid "false alarms", in which bona fide customers or other innocent persons are wrongly pointed out as carrying stolen or contraband goods through the portal, since this not only leads to immediate wrongful embarrassment of the individual involved, but also is likely to cost the merchant or other proprietor the loss of substantial goodwill and, potentially, possible litigation by those claiming to be damaged by such incidents.
Accordingly, real progress satisfying both of the aforementioned requirements of high sensitivity attended by great selectively has been difficult to achieve and slow in coming. This conclusion is evidenced by the issuance of various patents over a long period of years, each asserting the achievement of improvements, but each followed in time by another patent directed to still a further improvement in a seemingly continuous sequence. By way of example, perhaps the most frequently-employed, and probably the most successful system concept, relates back to the often-noted French Pat. No. 763,681, of P. A. Picard, issued in 1934, in which the technological phenomenom is described as involving electromagnetic field perturbations resulting from the insertion or presence within the field of a piece of magnetic material. In particular, Picard noted the field effects created by the presence of highly-magnetic (high permeability) material such as permalloy, which creates the presence of a number of the higher-order odd harmonics of the fundamental frequency of the applied field (e.g., Picard referred to the presence of the ninth and eleventh harmonic). While a period of almost 50 years has elapsed since the appearance of this patent to Picard, various patents continue to issue from time to time asserting advances in Picard's theories and findings in the area of "pilferage detection" systems of the type noted hereinabove; for example, reference is made to a number of patents issued to Edward Fearon (including U.S. Pat. Nos. 3,631,442, 3,754,226, 3,790,945, 3,820,103, 3,820,104) and to Peterson (U.S. Pat. No. 3,747,086), Elder et al. (U.S. Pat. No. 3,665,449 and 3,765,007) as well as U.S. Pat. No. 3,983,552 to Bakeman. Indeed, a very recent such patent is that issued to Robert Richardson, U.S. Pat. No. 4,300,183, which is directed to and describes various attributes of the underlying concept relating back to Picard.
As stated above, the seemingly continuous advance in the general state of the art, as evidenced by the aforementioned patents, has undoubtedly provided new insights and improvements in the general level of the art, but requirements of truly satisfactory detection systems are very severe and demanding, and the need therefore continues to exist, and in some ways becomes even more pronounced, for truly reliable systems which will unerringly detect relatively small "marker" elements or indicia, while at the same time being essentially immune to a practically endless number of widely-varying metal devices, objects, articles, and components, all of which cause perturbations in the magnetic interrogation field, with resulting detection-actuating results being inevitably present.